The present invention lies in the field of antennas for land cellular radio communications networks, and it relates to a remote control method for adjusting the tilt of the radiation pattern relative to the horizontal of the antennas of a base station.
The invention also relates to an antenna fitted with means for adjusting its tilt and to a system for remote control of the tilt.
The technical field of the invention is active antennas (also known as xe2x80x9csmartxe2x80x9d antennas or as xe2x80x9cadaptivexe2x80x9d antennas) for networks for communicating with mobile telecommunications terminals.
When optimizing their networks, one of the problems encountered by cellular network operators is associated with monitoring the radio coverage provided by each base station. One of the means used for adjusting such coverage consists in modifying the tilt of the antenna (i.e. the angle at which its radiation pattern slopes relative to the horizontal) for the purpose of reducing (or sometimes increasing) the range of a base station; tilt modification is also used to reduce interference caused by a base station serving a remote cell in which the same spectrum resources are used in order to increase the capacity of the network.
At present, and in both cases, it is necessary to take action on site (which is expensive since it is necessary to use the services of an antenna-installation specialist), and to adjust the tilt of the antenna mechanically.
U.S. Pat. No. 4,249,181 (LEE) describes a cellular mobile radiotelephone mobile system in which radio frequencies are allocated to cells that are located adjacent to one another in order to form a network covering a service zone; at least one directional antenna system is provided in each cell; an antenna comprises a plurality of collinear dipoles fed in-phase; in order to tilt the axis of a lobe of the radiation pattern of the antenna in a vertical plane to beneath the horizontal, the antenna is fixed on a mast by means of a tilted mechanical support that may be fixed or adjustable, so as to increase the efficiency of coverage in the cell served by the antenna (relative to the situation in which the axis is horizontal).
Adjusting tilt by adjusting a mechanical antenna support requires on-site intervention which is expensive. It is also impossible with that system to adjust the radio coverage of a cell in accurate manner without multiple on-site interventions.
Such a mechanical device for adjusting the inclination of the antenna also suffers from the drawback of deforming the radiation pattern of the antenna in the horizontal plane.
It is also possible to use antennas in which the radiation pattern is tilted by phase shifting the signals fed to the individual antennas making up the antenna.
International application WO 98/21779(HUYNH), filed Nov. 12, 1997 and printed May 22, 1998, describes an antenna in which beam tilt is electrically variable; the antenna has radiating elements forming three groups of elements disposed along a screen; a phase adjustment mechanism is disposed between the second and third groups of antenna elements and comprises:
an inlet coupling element;
a moving coupling portion having one end pivotally mounted which is electromagnetically coupled to the inlet coupling element;
a semicircular transmission line portion which is coupled to a second end of the moving portion; and
a drive mechanism for the moving portion, which mechanism includes an electric motor that can be remotely controlled and monitored.
The electrical path lengths at the operating frequency are such that when the moving portion is in a middle position, they give rise to progressive phase shifting between the radiating elements which result in the radiation pattern being tilted through about 7xc2x0 below the horizontal; this tilt is adjustable in the range 0 to 14xc2x0 by displacing the moving portion (using the motor), thereby modifying the electrical path lengths, and consequently the relative phases of the signals delivered to the radiating elements.
That electromechanical phase shifting device is designed to make it possible to control remotely the tilt of the radiation pattern, in particular as a function of variations in demand level during holiday or weekend periods; the system suffers from certain drawbacks: it does not enable phase shifting to be varied quickly; in addition, its reliability is limited by the reliability of the electromechanical mechanism for modifying the electrical path lengths.
An object of the present invention is to provide apparatus for varying the tilt of the radiation pattern of such an antenna which is capable of being remotely controlled which is improved, and which, at least in part, remedies the drawbacks of known devices.
In a first aspect, the invention consists in providing an antenna for a land cellular telecommunications network, the antenna comprising radiating elements and feeder means for feeding said radiating elements with radio frequency (RF) signals; said feeder means include electronic phase shifter means for obtaining phase shifts between said signals so as to enable the tilt of the radiation pattern in a vertical plane to be adjusted remotely, in particular the axis of the main lobe of the pattern.
The electronic phase shifting means comprise one or more phase shifting cells each generally constituted by three components disposed successively in series: a splitter circuit; a static phase shifter circuit connected to one of two outputs of the splitter to deliver a phase shifted signal at its output; and a coupler having two inputs in quadrature.
The static phase shifter circuit is preferably constituted by an array of fixed phase shifter elements, each associated with a diode (or an RF relay) for switching the phase shifter element, with conduction of the diodes (or closure of the relays) being under the control of a digital control signal applied to the phase shifter circuit; the circuit preferably includes PIN type switching diodes.
By using a phase shifter circuit having two, three, or four phase shifter elements together with their associated diodes (or relays), a 2-, 3-, or 4-bit digital phase shifter is made available.
The invention thus provides a device that is very simple, accurate, very reliable, compact, of low cost, easy to control remotely, and capable of varying the tilt of the radiation pattern of an antenna in less than 1 microsecond (xcexcs), in particular when diodes are used, since the very short switching time thereof can be of the order of 10 nanoseconds (ns) to 100 ns.
The device of the invention also gives rise to little distortion of the radiation pattern in a horizontal plane.
In order to modify the tilt of the antenna, electronic devices (phase shifters) are inserted in the radio frequency feed circuit of the antenna for the purpose of causing the radiation pattern to tilt in a vertical plane. Such an electronic device can be remotely powered via the coaxial cable connecting the base station to the antenna. Controlling the electronic device (digitally or by analog means) makes it possible to adjust the value of the tilt to a value that has been predetermined by the operator (in steps of 1degree, for example). It is possible for control to be performed remotely from the base station, for example at the unit which controls and operates the cellular network. The installed remote control makes it possible to adjust tilt in real time without human intervention on the antenna.
The ability to adjust antenna tilt by means of an electronic device makes it possible in time division multiple access (TDMA) cellular networks (of the GSM type), to control radiation pattern tilt individually for each time slot (frame) by varying tilt at a frequency that is equal to the reciprocal of the TDMA frame duration, and thus to do so mobile terminal by mobile terminal, as a function of level or quality information received at the radio interface. Adjusting tilt for each ongoing call can be used in addition to controlling power so as to adjust the useful signal level and so as to reduce the level of an interfering signal situated within the aperture of the antenna.
An electronic device for phase shifting an elementary antenna is integrated within each antenna. Control of that device makes it possible to adjust antenna tilt. When control is performed remotely, an operator can adjust tilt without taking any particular action on the antenna. When measurements performed on the network show that tilt needs to be modified in order to reduce interference or to modify radio coverage, tilt can likewise be modified without extra cost.
If all of the antennas of a cellular network are fitted with the device, then there is no longer any need to install antenna tilting apparatus, nor is it necessary to specify to the antenna manufacturer, where appropriate, that an antenna should be supplied having a predetermined tilt value.
Most cellular network antennas are used in duplex mode (transmission and reception via the same antenna); the electronic device makes it possible to obtain identical electrical tilt on the up path and on the down path.
The electronic device included within the antenna-protecting dome is thus essentially constituted by phase shifters whose control signals can be conveyed by a coaxial cable that also carries the radio signals to be transmitted. Because control is performed in digital form, it is possible to integrate control in cellular network equipment after the antenna has begun to be used.
Thus, in another aspect, the invention consists in a method of controlling a plurality of antennas of the invention in which each antenna has delivered thereto a digital signal for controlling its electronic phase shifter means.
In preferred implementations of the invention:
the digital control signal is carried by a coaxial cable which also carries radio signals;
tilt value is adjusted periodically; and
said digital signal for varying the tilt of the radiation pattern is controlled at a frequency that is equal to the reciprocal of the duration of a TDMA frame so as to determine the tilt of the radiation pattern during each frame period for each telecommunications mobile terminal served by the antenna as a function of level or quality information concerning the signal received by the mobile terminal.